The invention relates to a method and apparatus for supporting and passing webs such as paper and board webs in a contacting manner in a paper/board machine in which the base web forming section is immediately followed by web finishing devices for immediate treatment of a web leaving the base web forming section.
With the increasing demands on higher web speeds and quality, well-behaved supporting and drying of a running web are becoming increasingly important in paper and board manufacture. Therefore, it is desirable to pass the web in a continuously or at least partly supported manner over a maximally long total distance of its travel in the web finishing sections. The technique used for web support also affects the drying process of the web. As web breaks occur more easily in a wet web than in a dry web, unsupported passages of the wet web over the distance from one support member to the next, before the wetted web is dried, represent potential points of web breaks. If the web can be supported in a contacting manner as long as the web is wet, the number of web breaks can be reduced. Simultaneously, the coating of the web may also be carried out at a higher moisture content and thus in a state of weaker tensional strength than has been possible in the prior art. As a result of processing the web at said higher moisture content, substantial savings can be attained in regard to both the overall energy consumption and investment costs of the dryer equipment. Further, as the specific drying capacity required in the different process steps for obtaining sufficient strength qualities of the web is not set any more, the moisture content of the web can be kept optimal with respect to the desired end product qualities throughout the manufacturing process.
In U.S. Pat. No. 5,547,509 is described an apparatus for supporting a paper web during the application step when the web is passed through the coating and doctoring installation. In this embodiment, the coating applicator may be, e.g., an application roll and the web is passed onto a support belt before it enters the nip of the applicator and, after application, is passed in a supported manner to the dryers. The function of the support belt is to reduce the number of web breaks in application and doctoring as well as along the web passage from the doctor blade to the dryers. The dryers are of the non-contacting type and the web is supported over its passage between the dryers by means of small-diameter support rolls. The supported portion of web travel covers the applicators only. Before entering the applicator and during drying, the web runs unsupported. Furthermore, as the application takes place using an applicator roll, the entry of the web to the applicator roll is very complicated, because the applicator roll (kissing roll) of a roll applicator must be arranged to rotate in a coating mix pan, which necessarily requires the applicator roll to be the lower roll in the roll nip and, as a result, the coated side of the web to be the underside of the web entering the applicator nip. obviously, passing the web to one applicator is simple along a straight path, but as the web is conventionally coated on both sides, reversing the web travel for coating the opposed side requires a complicated guide roll set in which the web cannot be supported by any practicable means using a support wire or belt.
In German patent publication no. 44 20242 is described an application method in which the web is coated in a film-transfer coater and then passed over the backing roll to a drying cylinder. Operating in conjunction with the backing roll, there is arranged an unrestrained type of dryer and from the backing roll the web is passed to a wire-supported dryer cylinder. While the wire in this embodiment is supportedly passed through the dryer cylinder section, the most critical passage with respect to web breakages, that is, the web travel within the area of the applicator and the first dryer unit, is passed only supported at the support rolls. Since the web has no continuous support when it is wet and hence has the lowest strength, this kind of web supporting offers no substantial contribution to web runnability and lesser number of web breaks.
It is an object of the present invention to provide a method and apparatus for implementing a fully-supported travel of a web running on a wire or belt through the coating applicator and also, at least partially supported, during subsequent drying.
The goal of the invention is achieved by passing the web to a coater station or other web treatment device in which the web will be wetted and, further, to at least the first dryer located next downstream to the coater in a continuously contactingly supported manner.
According to a preferred embodiment of the invention, the web is passed in a continuously contactingly supported manner from a dryer preceding a coater to a dryer located next to the coater in the downstream direction of web travel.
According to an embodiment of the invention, the web is passed from a dryer preceding the coater to a dryer located next to a coater in the web downstream travel direction with the help of a single continuous support means.
According to an embodiment of the invention, the web is passed to at least one coater with the help of the support means of the upstream preceding web treatment device.
According to the most preferred embodiment of the invention, the web is passed to a coater from the upstream preceding web treatment device in a supported manner and only partially dried.
The invention offers significant benefits.
Continuous belt or wire support can effectively reduce the number of web breaks occurring in the manufacture of paper grades having a low basis weight. Additionally, the support embodiment according to the present invention based on belt-supported web travel permits the web to be passed to a coater only partially dried, because this arrangement eliminates the risk of breaks in a wet web. In this context, the term partially dried web must be understood to refer to a web having a moisture content essentially higher than that of the finished end product. In this manner, the drying process of the base web and its coating in the manufacture of a paper/board grade can be optimized so as to achieve a maximally high quality of the finished end product. As a result, the coating process can be carried out at the optimum moisture content with respect to the desired qualities of the coated paper/board product, specific energy consumption and the drying requirements of the base web and its coating, as well as with respect to the investment cost of drying capacity and the overall efficiency of the manufacturing process. By contrast, in prior-art embodiments it has been mandatory to pass the base web in a rather dry state into the coater in order to avoid web breaks. In practice, this has required the moisture content of the base web leaving the papermaking machine to be substantially equal to that of the finished end product.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.